The present invention relates to absorption heat transfer plants and in particular to a control system therefor.
Absorption refrigeration plants are well known in the refrigeration art and have been used in a wide variety of refrigeration applications. Examples of such plants are those using water as a refrigerant and those using ammonia as a refrigerant.
An absorption refrigeration plant comprises a concentrator, a condenser, an evaporator, and an absorber. In the concentrator, a dilute solution of absorbent and refrigerant is heated, thereby releasing refrigerant. The refrigerant vapor flows to the condenser, where the vapor is cooled and thereby condensed. From the condenser, the liquid refrigerant flows through an orifice into the evaporator. The evaporator is at a lower pressure than the condenser, thus a portion of the liquid refrigerant vaporizes, cooling the remaining liquid. The cooled liquid refrigerant cools the system fluid by direct heat transfer, which causes additional liquid refrigerant to vaporize. The cooled system fluid is used to cool an external refrigeration load. The refrigerant vapor flows to the absorber, where the vapor is absorbed by a relatively undiluted solution of absorbent and refrigerant, which dilutes the solution. The dilute solution is then pumped to the concentrator, completing the cycle.
A refrigeration absorption plant is generally subjected to a variable refrigeration load and variable external conditions, such as ambient air temperature. An automatic control system is generally provided to accomplish stable, efficient operation of the plant.
An additional consideration in absorption refrigeration plants which use water as a refrigerant is that if the refrigerant is allowed to freeze, the operation of the plant will be disrupted and the plant possibly damaged. The lowest temperature liquid refrigerant occurs in the evaporator. Thus, in the past, such plants were designed to maintain the temperature of the liquid refrigerant in the evaporator substantially above 32.degree. F. (generally between 40.degree. and 42.degree. F.) at full capacity to provide a safety margin against freezing the refrigerant.
Another consideration in absorption refrigeration plants which use water as a refrigerant arises due to the fact that to operate at optimum efficiency, the interior of such plants must be maintained at very low pressures, typically approximately 1 psia or below. Due to leakage and other causes, the pressure inside such plants tends to increase above optimum. A vacuum pump is used to return the plant's internal pressure to optimum. In the past, the process of pumping the internal pressure down to the optimum level was controlled manually. That is, a worker positioned several valves and turned on the vacuum pump, then reversed the process when the plant's internal pressure was within limits.
U.S. Pat. No. 4,505,123 (Kusakabe et al) discloses a control system in which the rate of flow of liquid refrigerant from the condenser to the evaporator is controlled in response to any of the following:
1. the temperature of the heat supplied to the concentrator, PA1 2. the temperature of the liquid refrigerant entering the evaporator, PA1 3. the temperature of the solution of absorbent and refrigerant, or PA1 4. the pressure of the refrigerant vapor in the absorber.
U.S. Pat. No. 4,706,464 (Kreutmair) discloses a control system in which the heat supplied to the concentrator is controlled in response to the temperature of the refrigerant vapor leaving the concentrator and the flow of liquid refrigerant from the condenser to the evaporator is controlled in response to the temperature of the refrigerant vapors at the outlet of the evaporator.
U.S. Pat. No. 4,894,988 (Kaneko et al) discloses a control system in which liquid refrigerant in the condenser may be diverted directly to the absorber when the temperature of cooled system water leaving the evaporator decreases below a preset temperature.
The Trane Company of La Crosse, Wis., manufactures an absorption refrigeration machine in which an electro-pneumatic control system controls the heat supplied to the concentrator in response to the temperature of the system fluid, the fluid which cools an external load. The Trane control system also controls the heat supplied to the concentrator in response to the temperature of a fluid which is used to cool the machine's absorber and condenser. The Trane control system is described in Operation-Maintenance Manual, Trane Company, La Crosse, Wis., 1983.